1. Field of the Invention
This invention relates generally to a digital circuit for controlling the gain of an amplifier stage. In particular, the invention concerns a circuit that is effective in preventing distortion phenomena due to clipping (i.e., dynamic saturation of an amplifier) in a receive channel for voice signals in digital form.
The field of application of the invention pertains, particularly but not exclusively, to digital telephone and cordless telephone equipment connected to integrated service digital networks. The description which follows makes reference to that field of application for simplicity of illustration only.
2. Discussion of the Related Art
In this type of communications network, analog voice signals are converted to digital signals by sampling. The digital signal, which for example may be coded with 8 bits, contains information as to both the amplitude (bits 7-1) and the sign (bit 0) of the sampled analog signal.
Digital telephone equipment of medium-high level using the PCM technique requires final amplifiers which can drive a speaker with sufficient power to allow "room" listening, even from a distance of a few meters from the telephone set. In general, the digital and analog circuits present in the signal path are designed to supply the speaker with the highest possible power at a low (e.g. &lt;0.2%) distortion when the input is a maximum amplitude signal. However, it is unlikely that a voice signal can attain and retain for some time a level corresponding to the maximum amplitude envisaged. Thus, the level of a standard phone call may be found low even at the highest volume setting of the called set.
On the other hand, if both the digital and analog circuits present in the signal path are designed to provide maximum output power with low distortion at a medium level of the input signal--leaving it for the subscriber to select a lower sound volume--then a sufficiently high level of telephone talk can be obtained even with the speakerphone operated. At above-medium level of the input signal, however, distortion problems due to clipping are encountered. Thus, the channel ought to be provided with an anti-clipping circuit affording, when required, maximum output power and low distortion. To fill that demand, state-of-art proposals include approaches of the analog type and of the mixed analog-digital type, which both make ample use of external components. Such approaches involve, however, high power consumption and increased costs.